1. Technical Field
The present invention relates to a method and apparatus for gasifying combustibles in a fluidized-bed furnace.
2. Related Art
It has been desired in recent years to incinerate wastes such as municipal wastes or plastic wastes that are produced in large quantities for reducing their volume and effectively utilizing their waste heat when the wastes are incinerated. Since incineration ash generated when the wastes are incinerated usually contain heavy metals, it is necessary to stabilize those heavy metals when the incineration ash is reclaimed in landfill sites.
It has theretofore been customary to treat a considerable amount of wastes such as municipal wastes, waste tires, sewage sludges, and industrial sludges with dedicated incinerators. Night soil and highly concentrated wastewater have also been treated with dedicated wastewater treatment facilities. However, large quantities of industrial wastes are still being discarded in an untreated state, thus causing environmental pollution.
As waste treatment technology suitable for environmental conservation to replace conventional incineration method, gasification and slagging combustion systems which combine gasification and high-temperature combustion have been developed, and some of them are about to be put to practical use. The inventors of the present application have proposed in Japanese patent application No. 8-331435 a method for treating wastes by gasification in which wastes are gasified at low temperature in a fluidized-bed gasification furnace, gaseous material and char produced in the fluidized-bed gasification furnace are introduced into a melting furnace and gasified at a high temperature, for thereby producing low calorific gas or medium calorific gas.
The fluidized-bed gasification furnace preferably comprises an internal circulating fluidized-bed furnace, and the melting furnace preferably comprises a swirling-type melt combustion furnace. It is most preferable to employ an internal circulating fluidized-bed furnace and a swirling-type melt combustion furnace in combination.
According to the proposed method, a fluidizing gas delivered into the fluidized-bed furnace comprises a central fluidizing gas supplied from a central furnace bottom into the furnace and a peripheral fluidizing gas supplied from a peripheral furnace bottom into the furnace. The mass flow of the central fluidizing gas is smaller than the mass flow of the peripheral fluidizing gas. An upward flow of the fluidizing gas in an upper peripheral furnace area is deflected toward a central furnace area by an inclined wall so that a moving bed where a fluidized medium (generally, silica sand is used) descends and is dispersed in the central furnace area, and a fluidized bed where the fluidized medium is intensely fluidized in the peripheral furnace area are formed. Combustibles supplied to the fluidized-bed furnace are gasified into a combustible gas while the combustibles are being circulated together with the fluidized medium.
The fluidizing gas delivered into the fluidized-bed furnace contains an amount of air which is equal to or less than 30% of a theoretical amount of combustion air required to combust combustibles. Incombustibles are discharged from the peripheral furnace bottom and classified, and sand obtained by classification is returned to the fluidized-bed furnace. Combustible gas may be produced by incomplete combustion of combustibles in the melt combustion furnace, and the produced combustible gas may be used as a town area gas or material for chemical synthesis. On the other hand, when the combustible gas and fine particles generated in the fluidized-bed furnace are completely combusted in the melt combustion furnace, exhaust gas having high temperature may be obtained, and the obtained gas may be supplied to a gas turbine for generating electric power and may also be used to generate steam with a boiler for generating electric power by using a steam turbine. As a result, a combined-cycle power generation system is capable for recovering energy.
In case of producing low calorific gas or medium calorific gas by incomplete combustion in the melt combustion furnace, the reaction in the melt combustion furnace is highly sensitive to gas conditions, at the inlet of the melt combustion furnace, including gas composition, pressure and temperature, and is particularly governed by the gas temperature. However, since municipal wastes as the material for the produced gas have different heating values, it is very difficult to obtain a gas having a stable temperature in the fluidized-bed furnace.
The temperature in the fluidized-bed furnace is controlled by adjusting the amounts of the peripheral fluidizing gas and the central fluidizing gas. When the combustibles are of a high-calorific material such as plastics, then the temperature in the fluidized-bed furnace which cannot be well controlled tends to be excessively high, and hence gas conditions of the generated gas including gas composition, temperature and pressure may be fluctuated.
The present invention has been made in view of the above problems, and it is therefore an object of the present invention to provide a fluidized-bed gasification method and apparatus which can suppress fluctuations in the temperature of a fluidized bed or a freeboard in a fluidized-bed furnace.
In order to achieve the above object, according to the present invention, a heat recovery region is provided adjacent to a fluidized bed with a partition wall interposed therebetween in a fluidized-bed furnace having a moving bed and a fluidized bed, and a heat exchanger such as heat transfer tubes in which a heating medium flows and an air diffuser are disposed in the heat recovery region. The amount of a fluidizing gas which forms the moving bed and the fluidized bed and the amount of a fluidizing gas supplied from the air diffuser in the heat recovery region are adjusted to allow a fluidized medium to flow over the partition wall into the heat recovery region, and the heat recovery rate from the fluidized medium is controlled to prevent the temperature of the fluidized medium from rising excessively.
Since the amounts of a peripheral fluidizing gas (or a second fluidizing gas) and a central fluidizing gas (or a first fluidizing gas) are adjusted to control the temperature of a combustion region to some extent, this temperature control in the combustion region is combined with the temperature control in the heat recovery region. In this combination, the temperature control in the combustion region (the moving bed and the fluidized bed) may be main temperature control and the temperature control in the heat recovery region may be auxiliary temperature control, or vice versa.
According to the present invention, a heat recovery region and a combustion region are provided in the fluidized-bed furnace, and the temperature of a fluidized bed or the temperature of a freeboard is controlled so as to be kept at a predetermined value by controlling the heat recovery rate in the heat recovery region.
According to an aspect of the present invention, the fluidized-bed furnace includes the heat recovery region and the combustion region for gasifying the combustibles, separated by a partition wall, upper and lower portions of the heat recovery region and the combustion region being interconnected; the combustion region is further divided into first and second areas adjacent to each other; the fluidizing gas supplied to the fluidized-bed furnace comprises a first fluidizing gas supplied as an upward flow into the furnace from an area near the first area at the furnace bottom, a second fluidizing gas supplied as an upward flow into the furnace from an area near the second area at the furnace bottom, and a heat recovery region fluidizing gas supplied to the heat recovery region; the mass flow of the first fluidizing gas is smaller than the mass flow of the second fluidizing gas, so that a moving bed where the fluidized medium descends and is dispersed is formed in the first area and a fluidized bed where the fluidized medium is intensely fluidized is formed in the second area, and combustibles supplied to the furnace are gasified into a combustible gas in the combustion region while circulating together with the fluidized medium; the fluidized medium in the combustion region flows over the partition wall into the heat recovery region, and the fluidized medium in the heat recovery region is returned from a lower portion of the partition wall into the combustion region; and the temperature of the fluidized bed or the temperature of the freeboard is controlled by adjusting the heat recovery rate in the heat recovery region with the heat recovery region fluidizing gas.
Preferably, temperature control in the fluidized-bed furnace comprises temperature control in the combustion region with the first fluidizing gas and the second fluidizing gas and temperature control in the heat recovery region, and the temperature control in the combustion region is a main temperature control and the temperature control in the heat recovery region is an auxiliary temperature control.
Preferably, temperature control in the fluidized-bed furnace comprises temperature control in the combustion region with the first fluidizing gas and the second fluidizing gas and temperature control in the heat recovery region, and the temperature control in the combustion region is an auxiliary temperature control and the temperature control in the heat recovery region is a main temperature control.
According to another aspect of the present invention, the fluidized-bed furnace is of a substantially circular in horizontal cross-sectional shape, and includes the heat recovery region at an outer peripheral region and the combustion region for gasifying the combustibles at a central region, separated by a partition wall, upper and lower portions of the heat recovery region and the combustion region being interconnected; the combustion region is further divided into a central area and a peripheral area; the fluidizing gas supplied to the fluidized-bed furnace comprises a central fluidizing gas supplied as an upward flow into the furnace from the central area at the bottom of the furnace, a peripheral fluidizing gas supplied as an upward flow into the furnace from the peripheral area at the bottom of the furnace, and a heat recovery region fluidizing gas supplied to the heat recovery region; one of the mass flow of the central fluidizing gas and the mass flow of the peripheral fluidizing gas is smaller than the other, so that a moving bed where the fluidized medium descends and is dispersed is formed in one of the central and peripheral areas of the furnace and a fluidized bed where the fluidized medium is intensely fluidized is formed in the other of the central and peripheral areas of the furnace, and the combustibles supplied to the furnace are gasified into a combustible gas while circulating together with the fluidized medium; the fluidized medium in the combustion region flows over the partition wall into the heat recovery region, and the fluidized medium in the heat recovery region is returned from a lower portion of the partition wall into the combustion region; and the temperature of the fluidized bed or the temperature of the freeboard is controlled by adjusting the heat recovery rate in the heat recovery region with the heat recovery region fluidizing gas.
Preferably, temperature control in the fluidized-bed furnace comprises temperature control in the combustion region with the central fluidizing gas and the peripheral fluidizing gas and temperature control in the heat recovery region, and the temperature control in the combustion region is a main temperature control and the temperature control in the heat recovery region is an auxiliary temperature control.
Preferably, temperature control in the fluidized-bed furnace comprises temperature control in the combustion region with the central fluidizing gas and the peripheral fluidizing gas and temperature control in the heat recovery region, and the temperature control in the combustion region is an auxiliary temperature control and the temperature control in the heat recovery region is a main temperature control.